This invention relates to implantable stimulators generally and more particularly to implantable cardioverters and defibrillators.
Early automatic tachycardia detection systems for automatic implantable cardioverter/defibrillators relied upon the presence or absence of electrical and mechanical heart activity (such as intramyocardial pressure, blood pressure, impedance, stroke volume or heart movement) and or the rate of the electrocardiogram to detect hemodynamically compromising ventricular tachycardia or fibrillation. For example, the 1961 publication by Dr. Fred Zacouto, Paris, France, entitled, "Traitement D'Urgence des Differents Types de Syncopes Cardiaques du Syndrome de Morgangni-Adams-Stokes" (National Library of Medicine, Bethesda, MD) describes an automatic pacemaker and defibrillator responsive to the presence or absence of the patient's blood pressure in conjunction with the rate of the patient's electrocardiogram to diagnose and automatically treat brady and tachyarrhythmias.
Later detection algorithms proposed by Satinsky, "Heart Monitor Automatically Activates Defibrillator", Medical Tribune, 9, No. 91:3, Nov. 11, 1968, and Shuder et al. "Experimental Ventricular Defibrillation with an Automatic and Completely Implanted System", Transactions American Society for Artificial Internal Organs, 16:207, 1970, automatically detected and triggered defibrillation when the amplitude of the R-wave of the electrocardiogram fell below a predetermined threshold over a predetermined period of time. The initial system proposed by Mirowski et al. in U.S. Pat. No. Re 27,757, which similarly relied upon the decrease in the amplitude of a pulsatile right ventricular pressure signal below a threshold over a predetermined period of time, was abandoned by Mirowski et al. in favor of the rate and/or probability density function morphology discrimination as described in Mower et al., "Automatic Implantable Cardioverter-Defibrillator Structural Characteristics", PACE, Vol. 7, November-December 1984, Part II, pp. 1331-1334.
More recently, others have suggested the use of high rate plus acceleration of rate or "onset" (U.S. Pat. No. 4,384,585) with sustained high rate and rate stability (U.S. Pat. No. 4,523,595). As stated in the article "Automatic Tachycardia Recognition", by R. Arzbaecher et al., PACE, May-June 1984, pp. 541-547, anti-tachycardia pacemakers that were undergoing clinical studies prior to the publication of that article detected tachycardia by sensing a high rate in the chamber to be paced. The specific criteria to be met before attempting tachyarrhythmia termination by pacing involved a comparison of the detected heart rate to a preset threshold, such as 150 beats per minute (400 millisecond cycle length) for a preselected number of beats. As stated above, other researchers had suggested the rate of change of rate or suddenness of onset, rate stability and sustained high rate as additional criteria to distinguish among various types of tachyarrhythmias.
In practical applications, a common approach has been to specify discrete rate zones for ventricular fibrillation and ventricular tachycardia, each defined by minimum rates or minimum R--R intervals. However, in some patients, ventricular tachycardia and supraventricular tachycardias including sinus tachycardias may have similar rates that make them difficult to distinguish from one another. For this reason, a more detailed analysis of the electrical waveforms associated with depolarization of the ventricles has often been employed to differentiate among various tachycardias.